The present invention is in the field of semiconductor manufacturing and more specifically relates to improvements in apparatus for handling semiconductor wafers as they are carried through a planarizing machine.
As supplied to the planarizing machine, the surface of a wafer may exhibit departures from flatness. Typically, 25 wafers are loaded into a cassette which is hand carried to a complex machine called a planarizer. The purpose of the planarizer is to render the front surface of the wafer flat to within a fraction of a micron. After each wafer has been rendered flat, the planarizer inserts it into a cassette. When the cassette is full, it may be removed from the planarizer and transported to any of a number of other processing machines.
The present invention cannot be fully appreciated without an understanding of the operation of the planarizer. Once the cassette containing typically 25 pre-processed wafers has been inserted into the planarizer, the operation of the planarizer in processing the wafers is completely autonomous. The operations are carried out under control of a computer that controls the application of power to various actuators in response to sensed inputs in a time-coordinated manner. In order for its lengthy program of operations to be carried out without mishap, it is important that means be provided to interrupt the accumulation of positional errors. It can also be appreciated that the wafers must be handled carefully to avoid scratching the processed surfaces which are so fragile that they are never again touched by human hands.
The present invention can best be understood by following a typical wafer as it is moved through the planarizer. The wafer is extracted from the cassette by a portion of the planarizer called a robot. The robot carries the wafer to a fixed location known as the load station. After the robot has deposited the wafer at the load station and has moved out of the way, a different part (called a spindle carrier) moves into position above the wafer and acquires it. The spindle carrier transports the wafer to a rotating polishing platen and rotates the wafer while holding it in contact with the rotating polishing platen until the wafer has been rendered flat. Thereafter, the spindle lifts the wafer from the polishing platen and transports it back to the load station where the wafer is rinsed. After the spindle carrier has deposited the wafer at the load station and has moved out of the way, the robot picks up the polished wafer and carries it to a cassette and deposits the wafer in the cassette. Throughout these operations, the wafer is maintained in a horizontal position with the side that is to be polished facing downward. That downward facing side of the wafer is also referred to as the front side of the wafer. The path of an individual wafer through the planarizer will be depicted in greater detail below.
Upon reflection it will be seen that the main purpose of the load station is to accurately center the wafer into the carrier notwithstanding the errors in their relative positions as they enter the load station.
In addition to centering the wafer with respect to the carrier, the load station performs a number of useful ancillary functions. The load station includes provision for sensing the presence of the wafer at the load station, for rinsing the wafer, for lifting the wafer into the carrier, and for supporting the processed wafer on cushions of water to avoid direct contact. Thus, the load station serves several other functions in addition to centering the wafer with respect to the carrier.
The process of centering the wafer with respect to the spindle carrier must be completed before the spindle carrier acquires the wafer; once acquired, the wafer is drawn against the vacuum pad of the spindle carrier and cannot be shifted laterally. To guard against premature acquisition, at the end of the centering process, when the wafer is at its lowest position, a space is left between the upper surface of the wafer and the vacuum pad of the spindle carrier. Some way is needed to elevate the centered wafer into contact with the vacuum pad without contacting the fragile lower surface of the wafer.
In accordance with the present invention this is accomplished by the use of an unusual type of nozzle located beneath the center of the wafer. The nozzle extends telescopically upward to provide an upwardly-moving cushion of purified water that lifts the centered wafer up into contact with the vacuum pad of the spindle carrier, while preventing contact between the nozzle and the fragile lower surface of the wafer.
The nozzle includes a stationary hollow cylinder within which a vertically movable spool slides in a loose sliding fit. In its lowest position, the spool is spaced from the closed bottom of the cylinder, and the purified water is supplied under pressure to the space. A passage extends vertically through the spool. In the absence of a wafer, the spool remains in its lowest position and the water is discharged at the upper end of the spool from the passage.
As the wafer reaches its lowest position at the conclusion of the centering process, the wafer closely approaches the upper end of the spool but does not make contact with it. However, the wafer partially impedes the discharge of the water, thereby increasing the pressure in the space below the spool. This increased pressure, acting on the lower end of the spool, overcomes the weight of the spool and the weight of the wafer, which is supported on the liquid cushion that prevents the nozzle from making contact with the wafer. As the spool moves upward it lifts the wafer on the liquid cushion until the upper surface of the wafer makes contact with the vacuum pad on the underside of the spindle carrier. In the process of elevating the wafer, the spool also rinses the lower side of the wafer, the rinse water flowing outwardly from the center of the wafer.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which a preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.